Alterations in the normal sequence of cellular differentiation in the developing brain can lead to abnormal circuitries which may be associated with major mental disorders. Cellular organization and differentiation may follow several courses leading to primarily layered (e.g., cerebellar cortices) or nuclear (cell groupings within the thalamus and hypothalamus) structures. This proposal concentrates on the formation of nuclear structure using the ventromedial nucleus of the hypothalamus (VMH) as a model system. The VMH is a key nucleus for regulating homeostatic, neuroendocrine, and behavioral functions. Cells in the VMH become part of a complex neural circuitry based on their ability to migrate to appropriate positions within the nucleus characterized by neurochemical environment, phenotype of neighboring cells, and the pattern of anatomical connections. The VMH is within a region where gonadal steroid hormones dramatically influence development and where hormone-concentrating cells dramatically affect physiology and behavior. This proposal exploits recent technical advances in our laboratory to study the migration of cells in the hypothalamus during development. In particular, our model system allows the formation of nuclei in vitro and is therefore easily accessible to live observation and manipulation. Our recent finding of a unique relationship of the neurotransmitter GABA to the developing VMH has led to hypotheses that will be tested through pharmacological manipulations. We will take particular advantage of mice for which a single gene deletion (the nuclear orphan receptor, steroidogenic factor-1) leads to failure of VMH formation. Three major questions to be addressed include (1) how are cells organized into the VMH?, (2) what role does the neurotransmitter GABA play in VMH formation?, and (3) how do gonadal steroid hormones influence VMH organization? It is hypothesized that mechanisms in addition to radial glial guidance contribute to VMH organization and that GABA and gonadal hormones play a role in the organization process. We will utilize in vitro time-lapse live video microscopy to follow the migration of labelled cells in brain slice preparations from embryonic mice. Additional animals will be used in parallel to test hypotheses in vivo using immunocytochemical (ICC) studies to provide an independent index of migration and organization. The proposed studies will bring us closer to determining the pattern-forming steps involved in the establishment of sexual differences in neural morphology and the actions of gonadal steroids on the developing nervous system.